为什么MySQL不建议使用NULL作为列默认值？

MySQL
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2024-01-09
MySQL字符集及其排序规则
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今天来分享一道美团高频面试题，5 分钟搞懂“为什么 MySQL 不建议使用 NULL 作为列默认值？”。

对于这个问题，通常能听到的答案是使用了NULL值的列将会使索引失效,但是如果实际测试过一下,你就知道IS NULL会使用索引，所以上述说法有漏洞。

着急的人拉到最下边看结论

前言

NULL值是一种对列的特殊约束,我们创建一个新列时,如果没有明确的使用关键字not null声明该数据列,MySQL会默认的为我们添加上NULL约束。

有些开发人员在创建数据表时,由于懒惰直接使用Mysql的默认推荐设置.(即允许字段使用NULL值).而这一陋习很容易在使用NULL的场景中得出不确定的查询结果以及引起数据库性能的下降。

介绍

NULL并不意味着什么都没有,我们要注意 NULL 跟 ''(空值)是两个完全不一样的值，MySQL中可以操作NULL值操作符主要有三个。

  • IS NULL
  • IS NOT NULL
  • <=> 太空船操作符,这个操作符很像=,select NULL<=>NULL可以返回true,但是select NULL=NULL返回false.
  • IFNULL 一个函数.怎么使用自己查吧…反正我会了

NULL通过任一操作符与其它值比较都会得到NULL,除了<=>.

(root@localhost mysql3306.sock)[zlm]>create table test_null(
    -> id int not null,
    -> name varchar(10)
    -> );
Query OK, 0 rows affected (0.02 sec)

(root@localhost mysql3306.sock)[zlm]>insert into test_null values(1,'zlm');
Query OK, 1 row affected (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>insert into test_null values(2,null);
Query OK, 1 row affected (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select * from test_null;
+----+------+
| id | name |
+----+------+
|  1 | zlm  |
|  2 | NULL |
+----+------+
2 rows in set (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select * from test_null where name=null;
Empty set (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select * from test_null where name is null;
+----+------+
| id | name |
+----+------+
|  2 | NULL |
+----+------+
1 row in set (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select * from test_null where name is not null;
+----+------+
| id | name |
+----+------+
|  1 | zlm  |
+----+------+
1 row in set (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select * from test_null where null=null;
Empty set (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select * from test_null where null<>null;
Empty set (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select * from test_null where null<=>null;
+----+------+
| id | name |
+----+------+
|  1 | zlm  |
|  2 | NULL |
+----+------+
2 rows in set (0.00 sec)

//null<=>null always return true,it's equal to "where 1=1".

NULL代表一个不确定的值,就算是两个NULL,它俩也不一定相等.(像不像C中未初始化的局部变量)

(root@localhost mysql3306.sock)[zlm]>SELECT 0 IS NULL, 0 IS NOT NULL, '' IS NULL, '' IS NOT NULL;
+-----------+---------------+------------+----------------+
| 0 IS NULL | 0 IS NOT NULL | '' IS NULL | '' IS NOT NULL |
+-----------+---------------+------------+----------------+
|         0 |             1 |          0 |              1 |
+-----------+---------------+------------+----------------+
1 row in set (0.00 sec)

//It's not equal to zero number or vacant string.
//In MySQL,0 means fasle,1 means true.

(root@localhost mysql3306.sock)[zlm]>SELECT 1 = NULL, 1 <> NULL, 1 < NULL, 1 > NULL;
+----------+-----------+----------+----------+
| 1 = NULL | 1 <> NULL | 1 < NULL | 1 > NULL |
+----------+-----------+----------+----------+
|     NULL |      NULL |     NULL |     NULL |
+----------+-----------+----------+----------+
1 row in set (0.00 sec)

//It cannot be compared with number.
//In MySQL,null means false,too.

任何有返回值的表达式中有NULL参与时,都会得到另外一个NULL值.

(root@localhost mysql3306.sock)[zlm]>select ifnull(null,'First is null'),ifnull(null+10,'First is null'),ifnull(concat('abc',null),'First is null');
+------------------------------+---------------------------------+--------------------------------------------+
| ifnull(null,'First is null') | ifnull(null+10,'First is null') | ifnull(concat('abc',null),'First is null') |
+------------------------------+---------------------------------+--------------------------------------------+
| First is null                | First is null                   | First is null                              |
+------------------------------+---------------------------------+--------------------------------------------+
1 row in set (0.00 sec)

//null value needs to be disposed with ifnull() function,what usually causes sql statement more complex.
//As we all know,MySQL does not support funcion index.Therefore,indexes on the column may not be used.That's really worse.

使用count() 或者 count(null column)结果不同,count(null column)<=count().

(root@localhost mysql3306.sock)[zlm]>select count(*),count(name) from test_null;
+----------+-------------+
| count(*) | count(name) |
+----------+-------------+
|        2 |           1 |
+----------+-------------+
1 row in set (0.00 sec)

//count(*) returns all rows ignore the null while count(name) returns the non-null rows in column "name".
//This will also leads to uncertainty if someone is unaware of the details above.

虽然select NULL=NULL的结果为false,但是在我们使用distinct,group by,order by时,NULL又被认为是相同值.

(root@localhost mysql3306.sock)[zlm]>insert into test_null values(3,null);
Query OK, 1 row affected (0.00 sec)

(root@localhost mysql3306.sock)[zlm]>select distinct name from test_null;
+------+
| name |
+------+
| zlm  |
| NULL |
+------+
2 rows in set (0.00 sec)

//Two rows of null value returned one and the result became two.

(root@localhost mysql3306.sock)[zlm]>select name from test_null group by name;
+------+
| name |
+------+
| NULL |
| zlm  |
+------+
2 rows in set (0.00 sec)

//Two rows of null value were put into the same group.
//By default,group by will also sort the result(null row showed first).

(root@localhost mysql3306.sock)[zlm]>select id,name from test_null order by name;
+----+------+
| id | name |
+----+------+
|  2 | NULL |
|  3 | NULL |
|  1 | zlm  |
+----+------+
3 rows in set (0.00 sec)

//Three rows were sorted(two null rows showed first).

MySQL中支持在含有NULL值的列上使用索引,但是Oracle不支持.这就是我们平时所说的如果列上含有NULL那么将会使索引失效。

严格来说，这句话对与MySQL来说是不准确的。

(root@localhost mysql3306.sock)[sysbench]>show tables;
+--------------------+
| Tables_in_sysbench |
+--------------------+
| sbtest1            |
| sbtest10           |
| sbtest2            |
| sbtest3            |
| sbtest4            |
| sbtest5            |
| sbtest6            |
| sbtest7            |
| sbtest8            |
| sbtest9            |
+--------------------+
10 rows in set (0.00 sec)

(root@localhost mysql3306.sock)[sysbench]>show create table sbtest1\G
*************************** 1. row ***************************
       Table: sbtest1
Create Table: CREATE TABLE `sbtest1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `k` int(11) NOT NULL DEFAULT '0',
  `c` char(120) NOT NULL DEFAULT '',
  `pad` char(60) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`),
  KEY `k_1` (`k`)
) ENGINE=InnoDB AUTO_INCREMENT=100001 DEFAULT CHARSET=utf8
1 row in set (0.00 sec)

(root@localhost mysql3306.sock)[sysbench]>alter table sbtest1 modify k int null,modify c char(120) null,modify pad char(60) null;
Query OK, 0 rows affected (4.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

(root@localhost mysql3306.sock)[sysbench]>insert into sbtest1 values(100001,null,null,null);
Query OK, 1 row affected (0.00 sec)

(root@localhost mysql3306.sock)[sysbench]>explain select id,k from sbtest1 where id=100001;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | sbtest1 | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

(root@localhost mysql3306.sock)[sysbench]>explain select id,k from sbtest1 where k is null;
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+
|  1 | SIMPLE      | sbtest1 | NULL       | ref  | k_1           | k_1  | 5       | const |    1 |   100.00 | Using where; Using index |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

//In the first query,the newly added row is retrieved by primary key.
//In the second query,the newly added row is retrieved by secondary key "k_1"
//It has been proved that indexes can be used on the columns which contain null value.
//column "k" is int datatype which occupies 4 bytes,but the value of "key_len" turn out to be 5.what's happed?Because null value needs 1 byte to store the null flag in the rows.

这个是我自己测试的例子：

mysql> select * from test_1;
+-----------+------+------+
| name      | code | id   |
+-----------+------+------+
| gaoyi     | wo   |    1 |
| gaoyi     | w    |    2 |
| chuzhong  | wo   |    3 |
| chuzhong  | w    |    4 |
| xiaoxue   | dd   |    5 |
| xiaoxue   | dfdf |    6 |
| sujianhui | su   |   99 |
| sujianhui | NULL |   99 |
+-----------+------+------+
8 rows in set (0.00 sec)

mysql> explain select * from test_1 where code is NULL;
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
| id | select_type | table  | partitions | type | possible_keys | key        | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | test_1 | NULL       | ref  | index_code    | index_code | 161     | const |    1 |   100.00 | Using index condition |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from test_1 where code is not NULL;
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
| id | select_type | table  | partitions | type  | possible_keys | key        | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | test_1 | NULL       | range | index_code    | index_code | 161     | NULL |    7 |   100.00 | Using index condition |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from test_1 where code='dd';
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
| id | select_type | table  | partitions | type | possible_keys | key        | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | test_1 | NULL       | ref  | index_code    | index_code | 161     | const |    1 |   100.00 | Using index condition |
+----+-------------+--------+------------+------+---------------+------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select * from test_1 where code like "dd%";
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
| id | select_type | table  | partitions | type  | possible_keys | key        | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | test_1 | NULL       | range | index_code    | index_code | 161     | NULL |    1 |   100.00 | Using index condition |
+----+-------------+--------+------------+-------+---------------+------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

总结

列中使用NULL值容易引发不受控制的事情发生,有时候还会严重托慢系统的性能.

例如:

  • 对含有NULL值的列进行统计计算,eg. count(),max(),min(),结果并不符合我们的期望值.
  • 干扰排序，分组,去重结果.
  • 有的时候为了消除NULL带来的技术债务,我们需要在SQL中使用IFNULL()来确保结果可控,但是这使程序变得复杂.
  • NULL值并是占用原有的字段空间存储,而是额外申请一个字节去标注,这个字段添加了NULL约束.(就像额外的标志位一样)

根据以上缺点,我们并不推荐在列中设置NULL作为列的默认值,你可以使用NOT NULL消除默认设置,使用0或者''空字符串来代替NULL。

MySQL字符集及其排序规则
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